Equipment for mixing a powder with a liquid

ABSTRACT

There is described a method of mixing a powder with a liquid comprising forming the liquid into a flowing film and dispersing the powder and directing it at the flowing liquid film so that it impinges thereon and mixes therewith. There is also described equipment ( 10 ) for mixing a powder with a liquid, comprising a mixing chamber ( 28 ), liquid inlet means ( 48 ) in the upper part of the chamber for directing the liquid on to an upper part of a supporting surface ( 31 ) in the chamber, the liquid inlet means ( 48 ) and the supporting surface ( 31 ) being so arranged that the liquid flows downwardly in adherence with the surface in use, and means for introducing the powder ( 13, 25 ) to the chamber ( 28 ) at a position spaced from the liquid inlet flow, for dispersing the powder within the chamber ( 28 ) and for directing it toward the said surface ( 31 ) so that it impinges on the liquid flowing down it, in use, and mixes therewith.

The present invention relates to the mixing of a powder with a liquid,particularly for enabling a solid reagent to mix and react with a liquidreagent.

Various methods of mixing solids and liquids are known. Many involve theagitation of the components in a rotary mixer combined with theirstirring by driven blades or other mixing equipment. Such equipment canbe very expensive to operate and can require the components to remain inthe equipment for a considerable length of time to ensure that mixing isthorough and any solid lumps that may be formed are broken down.

An object of the present invention is to provide a method for mixing apowdered solid with a liquid that is both efficient and can be put intopractice relatively cheaply so that it is suitable for use even withrelatively large quantities of the components.

Accordingly the invention provides a method of mixing a powder and aliquid comprising forming the liquid into a flowing film and dispersingthe powder and directing it at the flowing liquid film so that itimpinges thereon and mixes therewith. The film is preferably formed inadherence with a wall surface, the liquid preferably being supplied toan upper part of the surface so that it flows downwardly along thesurface under gravity and the powder is dispersed and directed at thewall to mix with the liquid.

The invention further provides, in another aspect, equipment for mixinga powder with a liquid, comprising a mixing chamber, liquid inlet meansin the upper part of the chamber for directing the liquid on to an upperpart of a supporting surface in the chamber, the liquid inlet means andthe supporting surface being so arranged that the liquid flowsdownwardly in adherence with the surface in use, and means forintroducing the powder to the chamber at a position spaced from theliquid inlet flow, for dispersing the powder within the chamber and fordirecting it toward the said surface so that it impinges on the liquidflowing down it, in use, and mixes therewith.

The advantage of the mixing method of the invention is that the liquid,being formed into a film presents a large surface area to the powderwhile the latter, being dispersed before it hits the liquid, will becaptured thereby with less tendency to form aggregates than when apowder and liquid are simply fed into a container and mixed. A uniformmixture can thus be obtained fairly readily without great expendituresof energy.

In preferred embodiments of the invention the film is formed in such away that its surface area expands as it moves away from the point atwhich the film is formed; this may be achieved by suitable shaping, forexample curving, of a surface on which it is formed. Moreover conditionsare preferably such that the film is turbulent, which, together with itslarge surface area, enhances the capture of the particles and mixingthereof within the liquid flow.

Although it is conceivable that the mixing might be carried out in anopen environment, it is preferably carried out in a closed chamber and,again, although the film might be formed at one side of the chamber withthe powder being directed at that side, it is preferred to employ acircular-section chamber with the film formed so as to flow downsubstantially the entire peripheral wall with the powder being dispersedfrom the central region and directed at the entire circumference.

Rotary feed inlets could be arranged to supply both the liquid and solidinto the chamber, the inlets rotating to cover the entire circumferenceand the powder possibly being blown into the chamber to disperse it.Much more preferably, however, the powder is fed through a central,axial inlet and is centrifuged to direct it at the circumferential wallwhile the liquid is supplied through an annular inlet or array of spacedinlets coaxially around the powder inlet. Centrifuge means provided inthe chamber may disperse the powder as well as directing it at the wallbut suitable gas jets may also assist the dispersal and/or direction ofthe powder.

Although the invention as defined above is applicable to the mixing ofany powder and liquid, it has been developed with particular concern tothe mixing of an hygroscopic powder with an aqueous liquid and, evenmore particularly, to the mixing of powder and liquid reagents whichreact together. In these circumstances it is important to keep the solidand liquid components separate until they are brought together in theflowing liquid film and, in particular, to keep the powder supply dry upto this point.

More particularly it is found, in practice, with the use of hygroscopicsolids, that it is both difficult and important to keep surfacesadjacent the powder inlet to the mixing chamber dry as any moisture inthis region can cause the powder to cake, the caking gradually buildingup with time. Eventually this provides a path for mixture to track backinto the powder supply duct, causing caking within the duct whichobstructs the supply itself and necessitates stoppage of the process forcleaning. At worst, in the case of hygroscopic materials which reactexothermically with water, this can result in substantial and evendangerous overheating, particularly if the moisture reaches a supplycontainer.

Accordingly, a further aspect of the invention comprises a method formixing an hygroscopic powder with an aqueous liquid comprising supplyingthe liquid to a chamber so that it flows in adherence with a surfacetherein, supplying the powder to powder delivery and dispersal meanswhich disperse the powder in the chamber and direct it at the saidsurface so that it impinges on the flowing liquid and mixes therewithwhile preventing liquid/solid contact on or adjacent the powder deliverymeans that could result in moisture tracking back to the powder supply,the supporting surface preferably comprising a wall of the chamberitself.

The chamber might be arranged with appropriate baffles to preventliquid/powder contact near the powder inlet but it has been foundpreferable to provide a dynamic seal between the powder inlet and partsof the chamber that might be contaminated by the liquid, the seal beingformed, for example, by a gaseous flow. In particular, in the preferredcircular-section chamber described above, an annular gas inlet isprovided coaxially around the powder inlet through which gas is suppliedto sweep moisture away from this region. The gas may be air depending onthe nature of the components to be mixed. This arrangement has the addedadvantage that the gas flow assists in the dispersal of the powderwithin the chamber.

In addition to the provision of the dynamic seal to preventliquid/powder contact at the powder inlet, the above mixing methodprovides for the formation of the liquid into a film that adheres to asurface in the chamber, preferably the chamber wall. Indeed steps arepreferably taken to ensure that the liquid does flow on the wall surfacewithout any splashing within the container which could result insplashing back to the powder inlet. For this purpose the delivery of theliquid to the wall surface is controlled both in quantity and directionso as to avoid splashing. To this end, the liquid may be delivered tothe surface in a direction substantially along the surface andconcordant with the desired direction of flow.

In a particularly preferred embodiment, the chamber has a domed upperwall and the liquid is fed substantially tangentially onto it from anupper inlet. The domed shape, conveniently although not necessarilyspherical, ensures the desired spreading of the film out from the inletto increase the surface area available to the solid particles. Thechamber wall may continue in a smooth curve from its maximumcircumference to an axial outlet but a frusto-conical shape is preferredin the lower part of the chamber to speed the fluid flow and to enhanceturbulence which improves the mixing of the solid and liquid components.

The conical taper also enhances vortical flow within the chamber whichmay be further promoted by the provision of swirler means within thelower part of the chamber or in an outlet duct therefrom. Such vorticalflow not only creates turbulence in the liquid but also causes adepression in the chamber which enhances any gas inlet flow around thepowder inlet and the dispersion and centrifuging of the powder. Inparticular, although this gas flow may be driven by auxiliary drivemeans with or without imparting rotation directly, it can be arrangedfor the gas to be drawn in solely by the depression caused by thevortical flow. Likewise, any swirler means provided may be driven torotate but, in practice, it is found that a static radial diffuserdevice achieves excellent mixing.

With reference now again to the liquid inlet flow, it was indicatedabove that this should preferably be controlled both in direction andquantity to prevent splashing. The quantitative flow may be, to someextent, controlled by appropriate valving in a supply duct but it isfound that splashing may occur particularly when the liquid supply tothe mixing chamber is either started or stopped. The liquid flow istherefore preferably controlled immediately at the inlet to the chamber.

For this purpose, regulable valve means may be provided at the inlet toadjust the size of the inlet aperture and/or the direction of the liquidflow through this aperture. The valve means may be regulable from theexterior of the equipment by manual, electronic or other suitablecontrols but most preferably are operated automatically in response to asensed flow of the liquid. Such sensing and control may, for example, beachieved electronically via appropriate sensors in the flow path but,for simplicity, the valve means are actuated by the liquid flow itself,the valve means being biased towards a condition in which a smallerliquid flow is directed, without splashing, onto an adjacent wallsurface over which it is to flow, and being movable by the flow itselfas this in creases, against the biasing force, to a position in whichthe larger flow is also directed onto the wall surface withoutsplashing.

The valve means preferably include a valve member movable between aposition in which the inlet aperture is of a minimum size to allow thesmaller liquid flow and a position of maximum size to allow the largerflow. The inlet means are preferably arranged, as indicated above, todirect the inlet flow substantially along the adjacent wall surfacerather than at a large angle to it, which would promote splashing, inall positions of the valve member.

In the preferred case of an annular inlet to a circular section chamber,the valve member is preferably also annular and mounted on a fixed partof the chamber surrounding the powder inlet. The valve member may bemovable, or expandable, radially of the chamber to reduce the inlet sizebut, in a preferred embodiment, is slidable axially of the chamberbetween its positions of use under the action of resilient biasingmeans.

In preferred embodiments of the invention, the powder supplied to themixing chamber is dispersed and directed at the peripheral wall at leastpartly by centrifuge means. Conveniently the powder is dropped from anupper inlet to the chamber onto the centrifuge means within the chamberpreferably closely below the powder inlet and including blades, vanes orother members extending from a rotary shaft which can break up any lumpsof powder as well as centrifuging it towards the peripheral wall. Therotary shaft preferably extends axially from the chamber to receivedrive from a suitable motor. Most conveniently it extends upwardly fromthe chamber through the powder inlet.

The powder may in some circumstances be fed solely under gravity to themixing chamber from a supply chamber above it but in most cases ametered supply is required in which case an auger or equivalent meansmay be provided between the supply chamber and the mixing chamber.

Metered supplies both of the solid and the liquid are particularlyrequired when the invention is employed for mixing solid and liquidreagents so that they can react but, more generally, the inventionfurther provides a method of reacting a solid reagent with a liquidreagent, including providing the solid in powdered form and mixing thepowder with the liquid under reaction conditions by forming the liquidinto a film that flows in adherence with a supporting surface anddispersing the powder and directing it at the flowing liquid film sothat it impinges thereon and mixes therewith.

The equipment described above is particularly useful for carrying outthis method as it enables the solid, in very finely divided form, to bebrought into contact with the liquid film so that intimate contactbetween the two media is achieved very quickly and over a very largesurface area which promotes very rapid reactions. Moreover, by virtue ofthe turbulent conditions that can be achieved in the liquid film, theliquid surface is continuously renewed so as further to promote captureof solid particles while reactions can continue within the body of theliquid as the solid particles are retained therein. Depending always onthe nature of the reagents themselves, the flow rates of the tworeagents can be adjusted to ensure that the reaction at least nearscompletion within the mixing chamber and within a very short stay time.

The equipment described above also lends itself readily to the carryingout of a reaction involving oxidation of one of the components byoxygen. Accordingly, the invention further provides a method of reactinga solid reagent with a liquid reagent and simultaneously oxidising acomponent that can be oxidised by oxygen, including providing the solidin powdered form, forming the liquid into a flowing film and dispersingthe powder with the aid of a gas flow containing oxygen whilesimultaneously directing the powder at the flowing liquid film so thatthe powder impinges on and mixes with the liquid, the reagents beingheld under such conditions that the oxidisable component is oxidised bythe oxygen in the gas flow and the solid and liquid reagents react inthe flowing liquid film.

As indicated above, the liquid inlet of the chamber of the invention mayconveniently be separated from the powder inlet by a dynamic air sealand this air may, in addition to helping to disperse the powder andpreventing moisture from tracking back to the powder source, also supplythe oxygen required for the oxidation reaction.

The equipment described above may include appropriate storage, deliveryand metering means for delivering the two components to the mixingchamber in desired relative proportions. Also it may include a receivingvessel for the mixture, from the chamber where, if necessary, thereaction may be completed before the finished product is stored ordischarged. Appropriate monitoring equipment for monitoring the finishedproduct and/or the starting components may also be provided togetherwith appropriate feed-back controls for changing conditions asnecessary.

The method and equipment described above has been devised particularlywith a view to treating acid mine discharges which often contain avariety of toxic metals, to enable liquid, specifically water, and solidproducts to be obtained which can be discharged safely into theenvironment or disposed of safely in appropriate dumps. To this end itis found possible to treat the acid discharge with a mixture of oxidesto neutralise it or raise its pH to a slightly alkaline value in acontinuous flow in the equipment described above, the reaction possiblybeing completed in ducts downstream of the mixing chamber, and to obtaina sedimentable suspension in which the metals are bound in non-toxicform. The invention further comprehends a method of treating minedischarges to render them safe and disposable as indicated above.

In a variant of the invention, the mixing method further comprehends astep of mixing additional liquid with the liquid/solid mixture formed inthe flowing film or, considered alternatively, a two step mixing processcomprising a first step in which a powder is mixed with a first quantityof liquid by forming the liquid into flowing film and dispersing thepowder and directing it at the flowing liquid film so that it impingesthereon and mixes therewith and a second step in which a furtherquantity of liquid is mixed with the mixture resulting from the firststep.

This variant method of the invention has the advantage that therelatively difficult process of mixing a solid quickly and thoroughlywith a liquid is achieved by the first step of the method of theinvention which can be carried out in the equipment described above,with appropriate devices being provided and measures being taken toensure proper mixing even of hygroscopic solids; even large volumes ofliquid can then be added easily without the need for expensive mixingequipment. In particular, although the initial solid/liquid mixture andadditional liquid may be fed into a common vessel provided withpower-driven agitators, such a solution would require an additionalpower input and it is found possible to achieve the required mixing byforcing the two flows to pass through a common duct provided with staticmixer means which cause turbulent flow and hence mixing.

Various mixer blades or baffles may be envisaged but, in combinationwith the preferred equipment described above, a radial diffuser devicesimilar to that used to enhance vortical flow in the primaryliquid/solid mixture, may be used.

The liquid/solid and liquid flows may simply feed into a common ductprovided with the mixer means but it is found that enhanced performanceis achieved if the liquid/solid mixture is supplied to outlet meanswithin a duct carrying the larger liquid flow, the flows being in thesame direction, and the duct having a Venturi restriction substantiallyaround the outlet means such that, in use, the depression caused by theflow through the Venturi constriction draws the liquid/solid mixtureinto the liquid flow. The outlet means may comprise an axial is outletfrom a duct carrying the liquid/solid mixture and/or peripheralapertures in an end portion of the duct. In addition to enhancing themixing of the solid/liquid and liquid flows, the Venturi effect mayprovide the required depression in the mixing chamber to cause air to bedrawn into it.

Two embodiments of the invention will now be more particularlydescribed, by way of example, with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic axial sectional view of equipment for mixing apowder and a liquid, shown in two parts 1 a and 1 b;

FIG. 2 is an enlarged sectional view of a detail of FIG. 1;

FIG. 3 is an enlarged sectional view of a further detail of FIG. 1;

FIG. 4 is an enlarged sectional view of a further detail of FIG. 1; and

FIG. 5 is a schematic axial sectional view of a variant of the equipmentof FIG. 1 fitted with auxiliary liquid-supply equipment.

With reference to FIGS. 1 to 4 of the drawings, the equipment showngenerally indicated 10 includes a main outer cylindrical casing 11arranged with its axis vertical and closed at the top by a cover 9. Anupper part 12 of the casing 11 defines the upper portion of a powderhopper 13; the lower portion of the hopper 13 is defined by afrusto-conical wall 14 sealed to the casing 11 around its upper,larger-diameter circumference 15 and tapering inwardly and downwardlytherefrom to a bottom opening 16 sealed to a concave cylindrical housing17.

The cover 9 supports a first motor 18 and a support and transmissionunit generally indicated 19 housed in an auxiliary casing 20 above themain casing 11. A second motor 21 is housed in a further chamber 22alongside the auxiliary casing 20 and the upper part of the casing 11and is connected to the support/transmission unit 19 via a belt 23. Themotor chamber 22 has an air inlet, indicated at 8, beneath the motor 21for admitting air to the chamber 22 and thence to the auxiliary casing20 to cool both the first and second motors 18, 21 and for further usein the equipment 10 as will be described below. The support/transmissionunit 19 will be described below in relation to FIG. 4.

The support and transmission unit 19 supports a drive tube 24 whichextends coaxially down through the hopper 13 and carries a n auger 25 atits lower end. This is located in the lowermost part of the hopper 13defined by the bottom portion of the frusto-conical wall 14 and thecylindrical housing 17 which closely surrounds the auger 25 to define afeed duct 25 a therewith. The auger-drive tube 24 also carries aplurality of mixer arms 26 spaced circumferentially around it atdifferent levels above the auger 25 but within the lower part of thehopper 13 and projecting outwardly and upwardly from the tube 24 itself.

The support/transmission unit 19 further supports a drive shaft 27 whichextends coaxially through the auger drive tube 24 and projects from thebottom end thereof into a mixer chamber 28 surrounding the outlet fromthe auger feed duct 25 a. A lower end portion of the drive shaft 27carries a plurality of flails 29 constituted by monofilament NYLON(Registered Trade Mark) lines secured in diametral through-holesarranged in a helical formation in the shaft 27 such that the two endportions of each line project by equal amounts in opposite directionsfrom the shaft 27.

The upper part of the mixer chamber 28 surrounding the outlet from theauger feed duct 25 a is bounded by a hemispherical wall 31 housed withinthe lowermost part 30 of the main casing 11 and arranged with itsconcavity facing downwardly. The lower circular edge of thehemispherical wall 31 is supported from the casing 11 by an annularflange 32 which also seals the gap between the wall 31 and the casing11. The lower edge of the hemispherical wall 31 continues into, or issealed to, a frusto-conical casing 33, coaxial with the casing 11, whichtapers downwardly to a lower, coaxial outlet to a tail pipe 34. A radialdiffuser 35 is mounted within the upper part of the tail pipe 34 beingcarried by a shaft 36. The diffuser 35 comprises a plurality of taperedfingers 35 a spaced radially around a mount carried by the shaft 36 andprojecting downwardly therefrom at an acute angle to the shaft axis. Thetail pipe 34 is of sufficient length to impart a substantial velocity toliquid flowing down it under gravity, in use, and thereby cause a vortexin the liquid flow for reasons explained more fully below.

The uppermost portion of the hemispherical chamber wall 31 defines anaperture coaxially surrounding the auger housing 17 and is surmounted byan integrally-formed annular wall 37 which has a slight outward conicaltaper. This wall 37 is joined to a further frusto-conical wall 38,having a much larger conic angle, and the outer periphery of which issealed to the main casing 11. The surfaces interconnecting thehemispherical wall 31, the annular wall 37 and the frusto-conical wall38 are smoothly curved.

As indicated above, the main cylindrical casing 11 of the equipment 10encloses two frusto-conical walls, the upper one 14, defining part ofthe hopper 13, and the lower one 38. Between these two walls 14, 38, thecasing 11 also supports a frusto-conical partition 39 having anintermediate conic angle and tapering in the same direction. Thisintermediate frusto-conical partition 39 is connected to the outercasing 11 via a radial flange 40 at its larger end while its smaller endsupports a dependent cylindrical duct 41 which extends coaxially betweenthe auger housing 17 and the annular wall 37 interconnecting thehemispherical chamber wall 31 and the frusto-conical wall 38. The duct41 defines an inner, annular passage 42 between its inner surface andthe housing 17, this passage opening at its lower end into the mixerchamber 28 and at its upper end into a compartment 43 within the casing11 but outside the hopper wall 14. The duct 41 also defines an outerannular passage 44 between its outer face and the annular wall 37, thispassage 44 communicating at its upper end with an annular chamber 45formed within the casing 11 between the frusto-conical wall 38 and thepartition 39 and at its lower end, with the mixer chamber 28. Thepassage 44 is shaped by the addition of a flow-deflector and valve unit,generally indicated 45 a, to the lower end of the duct 41 which will bedescribed more fully below in relation to FIG. 2.

The annular chamber 45 constitutes a liquid-inlet chamber for liquid tobe supplied to the mixer chamber 28 and, in use, receives liquid througha liquid-inlet pipe 46 sealed to an aperture in the main casing 11. Thecompartment 43, which communicates with the inner annular passage 42around the auger housing 17, on the other hand, constitutes an air-inletchamber and, in use, receives air through an air-inlet duct 47 sealed atits lower end to an aperture in the main casing 11 and at its upper endto an aperture in the auxiliary casing 20 so as to receive the coolingair for the motors 18, 21 ducted in through the inlet 8.

With reference now to FIG. 2 of the drawings, this shows in greaterdetail the flow-deflector and valve unit 45 a at the inlet to the mixerchamber 28. The unit 45 a is constituted by a two-part annularflow-deflector member 48 having a cylindrical inner face 48 a, whichco-operates with the outside of the duct 41, and a shaped outer face 48b facing the opposing surfaces of the annular wall 37 bounding theliquid-inlet passage 44 and the adjoining face of the hemispherical wall31. More particularly, the flow-deflector outer face 48 b has an upperfrusto-conical portion with a relatively small conic angle facing theannular wall 37 and a second frusto-conical portion with a large conicangle facing the hemispherical wall 31, these two portions beinginterconnected by a smoothly-curved surface. In addition, an annularknife-edge portion 49 projects beyond the outer edge of the main body ofthe deflector member 48, its surface continuing smoothly, but at aslightly different angle to the adjoining portion of the surface 48 b.The deflector member 48 is thus shaped to define a portion of thepassage 44 which tapers from its inlet end, nearer the inlet chamber 45,towards its outlet, indicated 50, adjacent the hemispherical chamberwall 31. Moreover it also changes the direction of this passage 44 fromgenerally axial and vertical adjacent the inlet chamber 45, to almosthorizontal at the outlet 50 such that a liquid flow therethrough, inuse, is directed along the hemispherical wall 31.

The flow-deflector member 48 is slidable on the outside of the duct 41and is urged upwardly therealong by one or more (only one shown in thedrawings) helical compression springs 51 located between a fixed flange52 attached to the lower end of the duct 41 so as to project radiallyoutwardly therefrom and a bottom surface 53 of the flow-deflector member48 itself. The flow-deflector member 48 is movable between upper andlower travel-limit positions in which the outlet gap 50 between theknife edge 49 and the opposing wall 31 are a minimum and a maximumrespectively: the deflector member is shown in full outline in aposition intermediate the upper and lower positions while the upper andlower travel limit positions of the knife edge 49 are indicated at 49Uand 49L respectively. Position 49U essentially ensures a clearance ismaintained between value 48 and chamber wall 31 in the nominally“closed” position of the valve. This clearance (approximately 1 mm) ismaintained by the accurate positioning of the lock nuts on the studswhich run through the compression springs 51 limiting the upward travelof the valve.

In use, the deflector member 48 assumes its upper position 49U at rest,under the action of the spring 51, but can be forced downwardly, againstthis action, when liquid is introduced into the inlet chamber 45 andflows down through the passage 44 into the mixing chamber 28. Theflow-deflector member 48 thus acts both to deflect the liquid flow alongthe wall 31 and as a variable-orifice valve member.

For convenience of manufacture, the deflector member 48 is formed in twoparts and an O-ring seal 54 is trapped in a chamber between them,against the duct 41, to prevent any seepage of liquid along this duct.The upper end of the deflector member 48 is protected by an annularcover plate 55 secured to the duct 41 above it and a further O-ring seal54 a is located in a recess in the outer face of the duct 41 in slidingcontact with the deflector member 48 further to reduce seepage betweenthe member 48 and the duct 41 and to prevent the ingress of abrasivesediment held in suspension in the liquid flow which might prevent thefree movement of the shielding valve under the influence of its springs51.

With reference now to FIG. 3 of the drawings, this shows the upper endof the equipment 10 including the auxiliary casing 20, thesupport/transmission unit 19, the upper ends of the auger drive tube 24and of the flail drive shaft 27 and their mounting on the cover 9.

The various parts of the assembly shown will not be described in detailas these and are not required for an understanding of the invention. Inbrief, the upper end of the auger drive tube 24 passes through anaperture 9 a in the cover 9 and is fixed to a coaxial tubular mount 60housed in the auxiliary casing 20. The upper end of the mount 60 issurrounded and supported by a fixed bearing housing 63, via a sealedbearing 61 between the bearing housing 63 and the mount 60, such thatthe mount 60 and the auger drive tube 24 are rotatable relative to thecover 9. The mount 60 receives rotary drive from the second motor 21(FIG. 1) and associated drive belt 23 via a pulley 62 fixed to its outercircumferential surface. A semi-dual rotary shaft lip seal 70 forms adust seal around the cover aperture 9 a, sealing the hopper 13 from thespace within the auxiliary casing 20. The bearing housing 63 further hasa circumferential wall 64 and an annular top plate 65. The upper end ofthe flail drive shaft 27 is rotatably supported in the aperture in thetop plate 65 and is connected to the motor 18 via a transmissionassembly 66. The auger drive tube 24 and flail drive shaft 27 thus arerotatable independently of each other and receive drive independentlyfrom their respective motors 21, 18.

A final feature of the equipment which may be noted from FIG. 3 is thefact that the flail drive shaft is coaxially surrounded by a tube 67housed coaxially within the auger drive tube 24. The tube 67 defines apassage 68 between its outer face and the auger drive tube 24 whichcommunicates at its upper end through apertures 86 in the bearinghousing 63 with the interior of the auxiliary casing 20 and, hence, withthe air inlet 8 of the chamber 22. The lower end of the passage 68 opensinto the mixing chamber 28 around the drive shaft 27 through apertures69 indicated in FIG. 4. The tube 67 is fixed against rotation, the driveshaft 27 being rotatable within it.

With reference finally to FIG. 4, this shows the portion of theequipment including the bottom end of the auger 25 carried on its drivetube 24. Again only parts particularly relevant to the invention will bedescribed.

FIG. 4 shows the bottom end of the auger housing 17 with the auger 25housed in the feed duct 25 a. The feed duct 25 a is open at its lowerend to define an annular outlet 25 b into the mixing chamber 28.

The flail drive shaft is rotatably supported within the auger 25 and itshousing 17 via a bearing assembly generally indicated 70 incorporatingthe apertures 69 which allow communication between the passage 68 andthe mixing chamber 28.

In use of the equipment 10 described above, powder is suppliedcontinuously to the hopper 13 through an inlet line (not shown) and isfed by the auger 25, driven by the motor 21, into the mixer chamber 28.Before the powder reaches the auger 25 it is stirred by the mixer arms26 which prevent log-jams which would interrupt the powder flow as itfalls into the mixer chamber 28; here it falls into the path of theflails 29, rotated by the motor 18, which break up any lumps of powder,which are in any case relatively soft, and distribute the powder as afine smoke in the mixer chamber 28.

The distribution of the powder at this stage is further assisted by airflows drawn in through the annular passage 68 around the flail driveshaft 27 and the annular passage around the auger housing 17. The airflow is driven solely by the vortex in the lower, frusto-conical portionof the mixer chamber 28 and in the tail pipe 34.

It is found in practice, in use of the equipment 10 for reacting acidmine discharges with oxides, as will be described below, that a tailpipe 34 with a fifteen foot (approximately 4.6 m) drop produces a vortexstrong enough to draw an adequate air supply through the equipment 10.It will be appreciated, however, that it may not be practical to providea fifteen foot drop in some situations in which case the air flow may beinduced by other means. For example an air compressor may be provided tosupply air to the inlet 8 to the equipment or a pump may be provided inthe liquid outlet pipe 34 to speed the liquid flow.

Simultaneously with the supply of powder, a liquid to be mixed therewithis supplied through the inlet duct 46, the liquid-inlet chamber 45 andthe annular passage 44, being directed through the narrow gap 50 betweenthe flow director knife-edge 49 and the chamber wall 31 and along thewall 31 to flow and spread down this wall 31 and onto the frusto-conicalwall 33. In this process the powder, flung centrifugally from the flails29, mixes and reacts with the liquid flow and is carried with it to thetail pipe 34.

More particularly, in order to achieve extremely effective mixing of thepowder with the liquid, the powder is introduced in the finest grainsize, as fine as talc, is broken into individual grains by the flails 29and simultaneously whisked into a swirling smoke by the air flows,primarily through the inner passage 68 but also through the surroundingpassage 42.

The mixing is further enhanced by the particular delivery of the liquidthrough the gap 50 onto the hemispherical wall 31 and the shaping ofthis wall; as the liquid flows down over this wall, the thickness of theflow decreases and its surface area expands so that an increasingsurface area is exposed to the turbulent smoke. Moreover the high degreeof drag between the thinning liquid flow and the chamber wall causesturbulence within the flow which in turn causes the surface layers to berenewed continuously so that further liquid is exposed to the powder andprevious surface layers, containing powder, are absorbed into the bodyof the liquid where any reaction that may be occurring continues.

It will also be appreciated that the turbulence in the film is enhancedas the liquid/solid/air mixture coalesces to flow down the tail pipe 34and passes through the radial diffuser 35 in the pipe 34. In practice itis found that the suction created below the diffuser fingers 35 aextracts air from the flow and creates a return flow of liquid in anupward direction from about 300 mm to 450 mm below the diffuser. Thishas a very positive effect in mixing the air and liquid as it causes amyriad of minute bubbles to form which then reverse direction again toflow down the pipe 34, causing thorough agitation and continued mixingof the solid and liquid so that any reactions still occurring in themixture can continue.

The equipment described above can thus achieve highly efficient mixingof any powdered solid and liquid but has further advantages with regardto the particular components for which it was developed. Specificallythe equipment is particularly suitable for treating aqueous liquidswhich contain some components , such as ferrous salts, which areoxidizable by the oxygen in air. Here the thorough mixing achieved withair in the apparatus is clearly advantageous. The equipment has evengreater advantages in the case of acid mine discharges which includecomponents oxidizable by air and others oxidizable by oxides,particularly when the oxides are solid but highly hygroscopic and notall soluble so that it is impractical to carry out the reactions insolution.

The hygroscopic properties of the reagent require stringent precautionsto be taken against moisture ingress in the delivery chain to the hopper13 but even present problems once the powder has been fed to the planthopper and metered by the auger to the point at which it is ready to bemixed with the acid discharge. Indeed any moist surfaces within themixing chamber will turn to paste any reagent powder deposited on them.The volume of this paste will then increase as further powder isdeposited on it until the spread of the paste accumulation extends toallow the moisture content to track to the powder source. This wouldprevent efficient distribution of the metered powder and at worst wouldcreate an exothermic reaction in the powder source, the metering auger.

To avoid such an occurrence it is essential that there are no moistsurfaces within the mixing chamber that can retain the powder and,having done so, allow the moisture to track back to the powder source.This is achieved in two ways; the chamber wall 31, 33 is continuouslyswept with acid and splashing on to components that cannot be swept isminimised as will be explained below; any component within the mixingchamber that cannot be swept with acid is isolated from the powdersource by a dynamic air gap of sufficient dimension to prevent bridgingby paste formation.

First of all, to ensure all surfaces within the mixing chamber arecontinuously swept with acid, the acid flow is controlled by thevariable-orifice flow deflector and valve unit 45 a. This ensures thatsufficient velocity is imparted to the acid flowing through the annularorifice is 50 to maintain a constant pressure of the flow over thechamber walls. The junction between the hemispherical wall 31, and theconical base section 33 (FIG. 1b) also imparts a sharp change indirection to the fluid flow such as to maintain the high pressure ofcontact and, in so doing, control the acid flow further to reduce anypossibility of cascading or splash.

The acid flow is initially controlled by an external feed valve. Whenthis valve is either opened or closed it will for a brief period reducethe acid flow to such a degree that all inertia is lost and, if noadditional steps were taken, the acid would cascade into the mixingchamber and, in so doing, splash acid in all directions, including thepowder source.

It is to prevent this occurrence that the orifice 50 through which theacid is introduced into the hemispherical part of the mixing chamber isfitted with the spring-loaded valve member 48. When there is no acidflow, the knife edge 49 of this valve member is sited approximately 1 mmclear of the surface of the mixing chamber so that even a gradualopening of the feed valve will provide an accelerated flow through theorifice 50 so that it reaches and adheres to the hemispherical wall 31of the mixing chamber. As the flow increases, the build up of pressurein the header tank above the valve unit 45 a will ease the valve member48 to widen the orifice 50, against the controlling spring pressure.With a gap of 8 mm around the periphery of the valve member 48, it hason test passed 850 GPM which is just under 1.25⁶ GPD. The valve member48 will open progressively with the flow pressure upon it until itreaches its maximum opening of 13 mm, but ensuring that at all times thevelocity imparted to the acid flow maintains the required degree ofcontrol. Clearly the dimensions of the valve members, the gap, thespring calibration and the flow rates may be varied according to theparticular intended usage of the equipment 10.

In use of the equipment described above for mixing/reacting solid andliquid components having consistent characteristics i.e. individualingredients or mixtures with ingredients in uniform proportions, thenthe liquid supply and auger feed rates can be set at the beginning of amixing process to meter the two components in appropriate relativeproportions, whether stoichiometric or otherwise, to achieve a desiredend result. In the case of acid mine discharges, however, the acidityand mineral contents of the liquid can be very variable and the rate ofsupply of the solid should preferably be varied to compensate. Inparticular, the pH of the discharge must be raised to a selected valueof at least pH7: the pH values of the mine discharge entering theequipment 10 and of the product leaving it are therefore monitored andthe speed of the auger varied in a PC fuzzy logic programme to ensurethat the final pH remains within predetermined limits of the desiredlevel.

With regard to acid mine discharges, these are often highly toxic andstreams may have high flow rates and are sometimes in ecologicallysensitive areas so that reliability of the treatment process isparamount. For this reason fail-safe systems are incorporated.

The main fail-safe system ensures the rapid shut-down of the equipment10, starting with the closing of the external feed valve. This can betriggered by an inadequate treated effluent pH signal or an unacceptablyhigh torque reading on the auger—indicating a potential breakdown intreatment.

The initial warning of low pH of the treated effluent would indicatethat sufficient reagent had been applied: this could be due to abreakdown in reagent supply to the auger feed hopper. This would bechecked by the PC which would respond if the low level transducer in thefeed hopper was calling for more powder. If “yes”, it would check thatthe mechanical powder conveyor was operating. If not, the system willshut down.

If the low level transducer was not calling for top up then it wouldindicate that there was sufficient powder available but this was notbeing passed by the auger.

The PC would check that the encoder recording auger speed was indicatingthat the auger was turning. If not, the system would shut down.

If the auger were turning but not delivering adequately, it wouldindicate a log jam in the powder hopper, preventing the auger frompicking up powder: in this instance an electric vibrator, not shown,fitted to the hopper would be operated for a brief period to free thejam. If no improvement was registered in the pH, the equipment would beshut down.

In a multiple module installation, each module being constituted by theequipment 10, when the PC is determining the cause of a malfunction orshutting down one module, remaining modules could open up to ensure thatthe overall level of treatment would be maintained.

Another aspect of design to enhance the reliability is the completeabsence in the mixing chamber of any form of mechanical agitator incontact with both acid and reagent, which would become encrusted withaccumulations of reagent paste. This would become a source of acidsplash creating the danger of acid reaching the reagent auger andpotential failure.

The degree of automation in the control and operation of the plant notonly enhances reliability but greatly reduces the labour costs ofoperating. The low labour requirement is also due to the simplicity ofthe system and the fact that large capacity installations may consist ofmultiple standard modules. A single standard allows adequate holding ofspares without the penalty of high inventory value.

The air requirements for motor cooling, reagent distribution and thepartial oxidising function, specifically converting the ferrous ironcontent in acid mine discharges to ferric iron, is provided byatmospheric air feeding the vortex in the plant. This conversion toferric iron by the oxygen in the atmospheric air reduces the amount ofsolid oxide reagent required for treatment. The vortex induced airflownot only eliminates the requirement for an energy-consuming aircompressor but also saves the capital and operating costs of suchequipment.

The elimination of an air compressor avoids the production of condensatewhen air is employed to distribute the reagent. The fitting of an airdrier to such a compressor only replaces the moisture produced withstatic electricity which adversely affects the powder distribution.

The equipment is very energy efficient for a given through-put with thepeak load of less than 3 kW and total energy consumption estimated to beless than 13 kWh per 24 hours of operation. The majority of the load isassociated with the aeromechanical conveyor used to lift the solidreagent into the hopper. In a commercial installation this lift wouldfeed up to six modules.

To treat the same effluent flow in a batch treatment process wouldtypically employ several process tanks and costly containment areas fortoxic sludge, occupying a considerable area.

The low energy requirement not only reduces the cost of the treatmentbut makes its application practical in many sites that require treatmentwhere electrical mains capacity is either limited or non-existent.

Each plant module is in itself very flexible in operation as it canreceive for treatment a very wide range of effluent flow rates and aninfinite variety of chemical characterizations. As all modules areidentical any number can be installed to cater for very large orseasonally varied flow rates.

In a multi-modular installation one module would normally be selected tomodulate with the remaining modules operating at a fixed delivery rate,this would prevent the tendency for the controls to hunt in operation.

Should an individual module require shutting down for routinemaintenance or repair the flow to the remaining modules would beincreased accordingly so that the overall treatment remains unaltered.

Any one of the group of modules could be selected to operate inmodulating mode.

In a particular embodiment of the invention, mine water containingarsenic, barium, cadmium, copper, manganese, iron, nickel, lead and zinccompounds was treated in the equipment 10 with an oxide mixtureavailable commercially and containing, calcium oxide, magnesium oxideand silica, (typically 50 ppm CaO, 150 ppm MgO, 800 ppm SiO₂+ componentscontaining SiO₂).

Treatment nt resulted in the reduction of metal concentrations in thesupernatant liquor by from 90-100%. Water quality appeared to improve ifthe water remained in prolonged contact with the sludge generated by theoxide application after leaving the equipment 10. Sludge obtained fromthe treated water consisted of fine-grained reddish-brown granulescontaining 10% white tubular calcite and gypsum crystals. Besidescalcite and gypsum, most of the sample consisted mainly of amorphousmaterial (presumably complex silicates and oxides of potassium,manganese, calcium, and iron) according to X-Ray diffraction.Chemically, the sludge consisted mainly of iron and calcium, with minoramounts of zinc, sulphur, silicon, aluminium, chlorine, magnesium,manganese, and potassium. Most of the transition metals were immobilisedin the solid sludge, resulting in greatly improved water quality.

More especially, the sludge obtained was subjected to a standard EPAtoxic chemical leach procedure to check its suitability for dumping.Specifically the pH of the sludge was measured at 7.5 indicating thatTCLP extraction solution No. 2 must be used. Due to the lack of volatilecompounds evident in this examination a zero head space TCLP procedurewas not employed. The following standard laboratory method was employed:

1. 5.0 grams of moist sludge material was placed in a 125 mlpolypropylene bottle.

2. 100.0 ml of TCLP extraction fluid #2 was added to the bottle.

3. The bottle was placed on a Burrel Wrist Action Shaker andcontinuously shaken for 18 hours.

4. The sample was filtered to 0.45 μm and subjected to ICP-AES analysisfollowing standard EPA protocol for ICP-AES analysis.

TCLP concentrations from the sludge passed U.S.EPA standards for toxicmetal leaching. None of the target metals (Ag, As, Ba, Cd, Cr, Hg, Pb.Se) were leached in excess of U.S EPA action levels for TCLP leaching.In addition, all the solid material which appeared to dissolve duringshaking was extracted by the 0.45 μm acetate filter indicating that thetoxic metal bearing particles within the sludge separated as suspendedsediment but did not dissolve during the leach procedure. This indicatedthat the sludge does not require further treatment and would not besubject to storage restrictions for disposal in the USA. Moreover, thedecant water above this sludge was sufficiently clear to satisfy U.Sfederal drinking water criteria.

Operating conditions for the test were as follows: Acid mine dischargewater (AMD) was fed to the equipment 10 via a 200 mm pipe from anunderground pump. The flow rate was monitored by a 200 mm magneticflowmeter situated downstream from a manually operated gate valve whichwas used to set the flow through the dosing plant. The solid oxidereagent (KB-1) was fed to the hopper 13 automatically by anaero-mechanical conveyor. The KB-1 was fed from the hopper by a variablespeed auger 25 to maintain precise delivery rates of reagent powder intothe mixing chamber 28.

From the flowmeter, the 200 mm pipe fed the AMD to the header tank ofthe plant, situated at the top of a 4.5 m (15 ft) high scaffold tower.The AMD was then accelerated by the variable control valve unit 45 awhich directed the flow radially from the centre of the hemisphericalmixing chamber so that it adhered to the concave walls.

The treated water leaves the tapered base of the mixing chamber via a200 mm diameter vertical tail pipe 34 leading to a maze consisting of anopen launder which, by turning the flow of treated AMD twice through180°, provided an excellent opportunity to observe the nature of thefloc and sediment formation in a very compact area.

The maze was dimensioned (9.54 m³) to allow a residence time of justunder 4.25 minutes at 38 Ls⁻¹ to enable the treated AMD time to reachfinal pH at the exit weir, where pH probes (pH2 or pH3) were positioned.An intermediate probe was positioned at the mid point of the first leg.

In a first trial the plant was operated with a continuous AMD flow of38LS⁻¹ and a wide range of oxide delivery rates to determine, byanalysis of the water samples, which range of delivery rates was themost economic whilst still satisfying the criteria of reducing theCadmium level to <0.001 ppm, the plan being to refine this range to aspecific rate in subsequent trials.

Trial 1

The first trial was started with a delivery rate of 2.1 gL⁻¹. The finalpH monitored at the outlet weir of the maze open launder was 12.5.

As this was excessive, the delivery was quickly reduced in steps until adelivery rate of 2.92 Kgmin⁻¹ at an auger speed of 40 rpm was reachedresulting in a final pH of 8.5.

As this was approaching the target are of 7.5 to 8.5 pH, it was decidedto reduce the delivery rate once more to 1.90 kgmin⁻¹ (26 rpm) reducingthe pH to 7.3 just below the target area. From this point the intentionwas to operate at progressively higher pH for periods long enough toallow the pH to stabilise, take a sample for water analysis, change thedelivery rate by a small increment and then repeat the process.

In this manner it was planned to cover a range of pH settings within thefocused area, then by reviewing the water analysis refine the process insubsequent trials with the object of running for more protracted periodsat each setting ensuring stable conditions and time to take sufficientsamples.

It was found that the first two samples taken at the depressed pH levelsreduced Cadmium from 0.049 ppm, to 0.015 ppm and 0.012 ppm respectively.The remaining six samples all reached EQS for Cadmium at 0.001 ppm orlower.

Such results for the first operation of the pilot plant at 38 LS⁻¹ (500GPM) were very encouraging, all the more so when it was found that asheared coupling had adversely affected the reagent mixing efficiency.

Trial 2

Following the result of the trial K1 it was decided to aim at an evenlower final pH by reducing the reagent delivery rate. Whilst it wasappreciated that half the trials may not reduce the Cadmium to 0.001 ppmit would narrow the target area for determining the most economicdelivery rate. The effect is shown in Table 1 below but are, to someextent, anomalous because of erratic pH readings.

TABLE 1 Final pH Lab analysis Cadmium/ Sample at weir pH ppm K2-1 7.76.7 0.001 K2-2 7.9 6.9 0.001 K2-3 7.9 7.3 0.001 K2-4 6.9 6.6 0.007 K2-57.6 6.8 0.006 K2-6 7.8 6.9 0.003 K2-1 was the lowest pH to have reducedthe Cadmium to 0.001 ppm.

Trial 3

The plant was run at two selected value of pH namely 8 and 9. Test K6-1at 9 pH reduced Cadmium to <0.001 ppm and still registered 8.9 pH at thetime of analysis. Test K6-2 run at 8 pH reduced Cadmium to 0.003 ppm andregistered 7.0 pH at the time of analysis, as shown in Table 2.

TABLE 2 Mean of pH at lab Sample pH2 & 3 Analysis Cadmium/ppm K6-1 9.18.9 <0.001 K6-2 8.0 7.0 0.003

Trial 4

The plant was run on five trials at 8.1 to 8.5 pH in 0.1 pH steps. Ascan be seen from Table 3, K8-2 and K8-4 both reduced the Cadmium levelfrom 0.052 ppm to >0.001 ppm.

TABLE 3 Mean of pH at lab Sample pH2 & 3 Analysis Cadmium/ppm K8-1 7.96.7 0.006 K8-2 8.3 7.6 <0.001 K8-3 8.4 6.9 0.003 K8-4 8.5 8.5 0.001

Trial 5

K11 was performed at a variety of delivery rates resulting in aprogressive reduction in the final pH values at the weir which reflectedthe progressive reduction in reagent delivery rate.

Results are shown in Table 4

TABLE 4 Mean of pH at lab Sample pH2 & 3 analysis Cadmium/ppm K11 8.8<0.001 K11-1 8.25 10.8 <0.001 K11-2 8.4 9.0 <0.001 K11-3 8.1 8.6 <0.001K11-4 8.4 8.5 <0.001 K11-5 8.0 8.3 <0.001 Environment 8.5 Not availableNot available Agency

As can been seen from Table 4, all six sample reduced Cadmium to <0.001ppm from 0.064 ppm. It is worthy of note that sample K11-5 was takenfrom a delivery rate of 0.8 gl⁻¹ or 1.8 Kg/min at 38 Ls⁻¹ (500 GPM).This was the lowest delivery rate of reagent to reduce the Cadmium to<0.001 ppm in this series of trials and was only 38% of the amount ofKB-1 required to reach the same criteria in prior CSMA laboratory tests,results of which are given in table 5.

TABLE 5 Solution KB-gL⁻¹ pH Cadmium Iron Manganese Zinc 2.1 9.19 <0.001In- 0.09 0.099 (AMD 3.04) sufficient sample 0.8 8.3  <0.001 0.3 1.4<0.01 (AMD 3.1) 

With reference now to FIG. 5 of the drawings, a variant of the equipment10 is shown, identical parts being indicated by the same referencenumerals. The equipment of FIG. 5 is in fact identical to that of FIGS.1 to 4 apart from a modification to the tail pipe 34 and the provisionof auxiliary liquid-supply equipment, generally indicated 70, around thefrusto-conical casing 33 of the mixer chamber 28 and the tail pipe, hereindicated 34 a.

The tail pipe 34 a in this embodiment is shortened and its lower endportion 71 tapers to an axial outlet 72. The tapered end portion 71 isalso perforated to provide a plurality of small outlet orifices 73. Thediffuser shaft 36 is supported within the tail pipe 34 a by means ofseveral struts 36 a extending between its lower end and the tail pipeend portion 71.

The auxiliary equipment 70 includes an inlet chamber 74 coaxiallysurrounding the casing 33 and having an upper cylindrical wall 75, anannular top closure wall 76 sealed to the casing 33 and a lower funnelwall 77 tapering from the lower edge of the wall 75 to an outlet duct 78coaxially surrounding the tail pipe 34 a. Inlet pipes 79 open into theupper part of the chamber 74 at circumferentially spaced positions; twosuch inlet pipes 79 are shown opening radially into the chamber 74 butthere may be several inlet pipes and their inclination to the chamberaxis may be varied. Currently, five inlet pipes 79 are envisaged, eachhaving a diameter of 200 mm and each equipped with appropriate valvingto enable the liquid supplied to the chamber 74 therethrough to beregulated.

The outlet duct 78 is fitted internally with a Venturi member 80 whichdefines a Venturi duct 80 a the narrowest portion of which coaxiallysurrounds the perforated end portion 71 of the tail pipe 34 a. The sizeand shaping of the Venturi member 80 may be varied according to thedesired fluid flows through the equipment in use. Currently the Venturimember is of fiber-glass reinforced resin while the remaining parts ofthe equipment are of stainless steel but other materials may be usedaccording to the materials to be mixed.

A final feature of the auxiliary equipment 70 is an additional radialdiffuser 81 supported coaxially within the outlet duct 78 downstream ofthe Venturi member 80 by means of a hollow shaft 82. The diffuser 81 issimilar to the diffuser 35 but of a larger size to match the largerdiameter of the outlet duct 78. Supplementary air supply ducts 83 extendthrough the wall of the duct 78 to open into the interior of the hollowshaft 82.

In use of the equipment 10 with the auxiliary equipment 70, liquid andpowdered solid are mixed together in the equipment 10 in exactly thesame way as described above and the auxiliary equipment 70 is used tomix a further quantity of liquid with the mixture formed in theequipment 10.

Additional liquid supplied through the inlet pipes 79 to the chamber 74flows down through the Venturi duct 80 a, thereby drawing the mixture inthe tail pipe 34 a through the perforations 73 to be mixed with theadditional liquid. The flow through the Venturi duct also causes therequired pressure reduction in the tail pipe 34 a which, in theembodiment of FIGS. 1 to 4, is caused by the length of the tail pipe 34itself.

The combined flow from the tail pipe 34 a and the outlet duct 78 thenreaches the additional radial diffuser 81 which ensures the thoroughmixing of the two flows. The mixing and any chemical reaction in theflow requiring the presence of oxygen are enhanced by air drawn into thecentre of the diffuser through the supplementary air supply ducts 83 bythe depression in the duct 78. The air supply through the ducts 83 isregulated to ensure that it does not increase the pressure in the mixingchamber 28 of the equipment 10 to a level above which the mixingefficiency is reduced: regulation is via a pressure-monitoring device inthe mixing chamber and diaphragm valve controlling the air supply.

In use of the equipment 10 and auxiliary equipment 70 for treating acidmine discharges, the provision of the auxiliary equipment 70 enables afar larger quantity of mine water to be treated per hour for a givensize of equipment 10 than is feasible with the equipment 10 alone,without loss of efficiency. Specifically, equipment 10 capable oftreating 10⁶ GPD (45⁶ liters per day) can treat up to 60⁶ GPD whenfitted with the auxiliary equipment 70, substantially equal efficiencybeing achieved throughout the range with the use of different Venturimembers 80 for the different flow rates.

In order to neutralise the additional mine discharge flow introducedthrough the auxiliary equipment 70, a larger proportion of oxide reagentmust be supplied to the mixing chamber 28 of the equipment 10 thanrequired to treat the mine discharge flow to the mixing chamber itself.The mixture leaving the mixing chamber 28 through the tail pipe 34 a isthus rich in oxides, the content being calculated to accord with theexcess required to treat the auxiliary flow through the duct 78.Thorough mixing of the two flows is achieved by means of the Venturi 80and diffuser 81 such that the reaction of the oxides with the minedischarge can go to completion. The equipment 10, 70 can thus treatextremely large quantities of liquid without the cost of additionalmoving parts or the increased power consumption required if theequivalent number of modules of the equipment 10 were used.

In view of the extremely large throughput achievable, it is envisagedthat it will be economical to use the enhanced equipment 10, 70 to raisethe pH of a mine discharge in several stages to enable the selectedprecipitation and recovery of different components of the waste. Thiswould, for example, enable various valuable metals, such as zinc, to berecovered from the waste, with the potential for reducing the overallcosts of treating wastes to meet standards for discharge into theenvironment.

What is claimed is:
 1. Apparatus for mixing powder with a liquid,comprising: a mixing chamber having an upper interior surface, saidupper interior surface progressively increasing in cross-section fromtop to bottom; liquid inlet means in an upper part of the chamber fordirecting the liquid on to an upper part of the upper interior surfacein the chamber so that the liquid flows as a film downwardly over and inadherence with the upper interior surface while simultaneouslyundergoing an increase in area; and means for introducing the powder tothe chamber at a position spaced from the liquid inlet flow, fordispersing the powder within the chamber and for directing the powdertoward said upper interior surface so that it impinges on the liquidflowing down said upper interior surface and mixes therewith as the areaof the film increases.
 2. Apparatus according to claim 1, wherein themixing chamber comprises a domed upper section, and said upper interiorsurface comprises an internally domed surface of said domed uppersection, said chamber having a nozzle positioned within said domedsection for feeding said liquid onto the internally domed surface in anupper region thereof so that said liquid forms a film of liquid flowingdownwardly over said internally domed surface with a concomitantincrease in area.
 3. Apparatus according to claim 2, wherein the nozzleis at least one of an annular nozzle, and an annular array of individualnozzles, positioned about a central axis of the said domed section andoperable to feed said liquid onto said domed surface over a complete areof 360 degrees.
 4. Apparatus according to claim 3, wherein the nozzlepositioning is such as to direct said liquid substantially tangentiallyonto an upper region of said domed surface.
 5. Apparatus according toclaim 3, wherein the nozzle comprises an automatic aperture controlwhich automatically adjusts nozzle aperture size between maximum andminimum values depending on a volume of liquid flowing through saidaperture, the larger the volume the larger the aperture.
 6. Apparatusaccording to claim 2, wherein the said mixing chamber comprises afrustoconical lower section axially aligned with and extendingdownwardly from the said domed section and down the internal wall ofwhich the liquid/powder mixture flows, upon leaving the said domedsection, and an exit conduit for the liquid/powder mixture positioned ata bottom of the frustoconical section.
 7. Apparatus according to claim6, wherein the exit conduit is a vertical conduit aligned on a commoncentral axis of the frustoconical and domed sections of the mixingchamber.
 8. Apparatus according to claim 7, which comprises a radialdiffuser positioned within the said exit conduit.
 9. Apparatus accordingto claim 6, wherein the exit conduit opens into a further mixing chambercomprising means for introducing a further quantity of liquid for mixingwith the liquid/powder mixture in said chamber, and another exit conduitfor recovering the liquid/powder mixture mixed with said furtherquantity of liquid from the further mixing chamber.
 10. Apparatusaccording to claim 9, wherein the further mixing chamber comprises anupper frustoconical section positioned above the lower, frustoconicalsection of the first mixing chamber, an upper section of said furthermixing chamber comprising said means for introducing a further quantityof liquid, and a lower straight section encompassing the exit conduitfrom the first mixing chamber, said lower straight section incorporatinga venturi adjacent a lower end of said exit conduit from the firstmixing chamber to accelerate said further liquid as it flows past saidlower end of the exit conduit chamber.
 11. Apparatus according to claim10, comprising a further radial diffuser located within the lowerstraight section of the further mixing chamber downstream of the venturiand said lower end of the first-mentioned exit conduit.
 12. Apparatusaccording to claim 1, wherein the means for dispersing and introducingthe dispersed powder into the mixing chamber comprises a feed conduitfor the powder extending downwardly into and terminating within a domedsection of the mixing chamber along the central axis thereof, and apowdered distribution means at the terminal end of the feed conduit,said powdered distribution means being operable to direct the powderparticles issuing from an end of the feed conduit towards saidinternally domed surface by means of a centrifugal force.
 13. Apparatusaccording to claim 12, comprising means for introducing an airstreaminto the mixing chamber along with said liquid and powder components.14. Apparatus according to claim 13, wherein the airstream introducingmeans operate to introduce said airstream as an annular stream of airsurrounding the terminal end of the powder feed conduit for the saidpowder.